Circuit Description
The DTC P0300 Engine Misfire Detected diagnostic monitors the engine for misfire. The powertrain control module (PCM) uses information from the crankshaft position (CKP) sensor and the camshaft position (CMP) sensor in order to determine when an engine misfire is occurring. By monitoring changes in the crankshaft rotation for each cylinder the PCM counts individual misfire events. The malfunction indicator lamp (MIL) illuminates when the misfire rate equals or exceeds a pre-determined count. A misfire rate that is high enough can cause the catalytic converter to overheat under certain driving conditions. The MIL will flash ON and OFF when the conditions for catalytic converter overheating are present.
Condition for Running the DTC
| • | The barometric pressure (BARO) is more than 75 kPa |
| • | The engine coolant temperature is more than -8°C (+18°F). |
| • | The intake air temperature is more than -8°C (+18°F). |
| • | The engine speed is less than 4,000 RPM. |
| • | The fuel tank level is more than 15 percent. |
| • | The throttle position (TP) sensor change is less than 1 degree/10 ms. |
Conditions for Setting the DTC
| • | The misfire rate at every 200 engine revolutions is more than a specified value in 2 or more cylinders. |
| • | The misfire rate at every 1,000 engine revolutions is more than a specified value in 2 or more cylinders. |
| • | The conditions are present for 5 seconds after engine start to 1 second from fuel shut-off. |
Action Taken When the DTC Sets
| • | The PCM flashes the MIL the first time catalytic converter damage occurs. |
| • | The PCM illuminates the MIL the second time emission thresholds are exceeded or the second time the diagnostic fails. |
| • | The PCM records the operating conditions at the time the diagnostic fails. This information is stored in the Freeze Frame buffer. |
Conditions for Clearing the MIL/DTC
| • | The MIL turns OFFafter 3 consecutively passing trips without a fault present. |
| • | A history DTC clears after 40 consecutive warm-up cycles without a fault. |
| • | Perform the scan tool Clear DTC Information function in order to clear the DTC. |
Diagnostic Aids
Check for any of the following conditions:
| • | If any DTCs other than misfire (P0300-P0304) are present, diagnose those DTCs first. |
| • | Any engine vacuum leaks |
| • | Engine overheating |
| Refer to Engine Overheating in Engine Cooling. |
| • | Contaminated fuel or inferior quality fuel |
| Refer to Alcohol/Contaminants-in-Fuel Diagnosis . |
| • | Inspect for the correct ignition timing and adjust as necessary. Refer to Ignition System Timing Adjustment . |
| • | An intermittent ignition system malfunction in the spark plugs, the ignition wires, or the ignition coils. Check the ignition system performance with an engine oscilloscope. |
| • | A damaged wiring harness. Inspect the applicable wiring harness for damage and repair as necessary. |
| • | An intermittent engine misfire may be caused by ignition system components that are susceptible to moisture. Spray water on the ignition system components and wiring in order to check for an engine miss or crossfire. |
Check for other intermittent conditions. Refer to Intermittent Conditions .
If a DTC P0300 is intermittent, driving the vehicle under the following conditions can verify whether the fault is present. Perform the scan tool Clear DTC Information function. Road test the vehicle while monitoring the DTC P0300 diagnostic on the scan tool under the Not Ran Since Code Cleared selection in the DTC Information menu. If a DTC P0300 appears in the Not Ran Since Code Cleared list, the P0300 diagnostic has not yet run. When the DTC P0300 does not appear in the Not Ran Since Code Cleared list, the P0300 diagnostic has run. If the MIL is NOT illuminated, and there is no Pending DTC Status in DTC Information, the P0300 diagnostic has passed. The DTCs MUST be cleared in order to view the Current Status of the Not Ran Since Code Cleared list. DO NOT forget that the Not Ran Since Code Cleared list only indicates that the test has run, not whether the test passed or failed. The DTC Information screen must be checked for Current or Pending status, in order to determine the outcome of the diagnostic test involved.
If a DTC P0300 cannot be duplicated, the information included in the Freeze Frame data can be useful in determining vehicle operating conditions when the DTC was first set. When the fault cannot be duplicated, wet the secondary ignition system with water and operate the vehicle under the conditions that the DTC set.
Test Description
The numbers below refer to the step numbers in the diagnostic table.
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The Diagnostic System Check-Engine Controls prompts the technician to complete some basic checks and store the Freeze Frame data on the scan tool if applicable. This creates an electronic copy of the data taken when the fault occurred. The information is then stored in the scan tool for later reference.
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This step checks whether the malfunction that caused the DTC P0300 is still present. Driving the vehicle under these conditions will verify whether the fault is present. After performing the scan tool Clear DTC Information function, the DTC P0300 diagnostic can be monitored on the scan tool under the Not Ran Since Code Cleared selection in the DTC Information menu. If DTC P0300 appears in the Not Ran Since Code Cleared list, the P0300 diagnostic has not yet run. When DTC P0300 does not appear in the Not Ran Since Code Cleared list, the P0300 diagnostic has run. If the MIL is NOT illuminated and there is no Pending DTC Status in DTC Information, the P0300 diagnostic has passed. The DTCs MUST be cleared in order to view the Current Status of the Not Ran Since Code Cleared list. DO NOT forget that the Not Ran Since Code Cleared list only indicates that the test has run, not whether the test passed or failed. The DTC Information screen must be checked for Current or Pending status in order to determine the outcome of the diagnostic test involved.
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This step checks the ignition coil for an internal fault.
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This step checks for the electrical signal from the PCM necessary for the operation of the fuel injector. If the injector test lamp does not illuminate the fuel injector control circuit is diagnosed for a faulty condition.
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Faulty fuel injectors can cause a misfire condition and a DTC P0300 to set.
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This step checks for a mechanical fault as the cause of the misfire condition. A low engine compression and an improper valve lash may also cause a rough idling condition.
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This step checks for faults in the CMP sensor and circuit that can cause incorrect ignition reference signals. Without continuous accurate reference signals, there is no dependable spark or fuel injector pulse. Erratic, missing, or incorrect engine reference signals can cause the engine to misfire.
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This step checks for a faulty CMP sensor signal rotor. Visually inspect the teeth of the signal rotor through the CMP sensor housing aperture for damage, foreign material, and mis-alignment.
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Clearing the DTCs by removing the DOME fuse also clears the PCM long term memory. Using the scan tool to clear DTCs leaves the long term memory active. DTC emitting diagnostics may not run to completion as expected unless the long term memory is cleared. Removal of the DOME fuse also clears the time display of the radio. The DTC P0300 diagnostic can be monitored on the scan tool under the Not Ran Since Code Cleared selection in the DTC Information menu. If a DTC P0300 appears in the Not Ran Since Code Cleared list, the DTC P0300 diagnostic has not yet run. When the DTC P0300 does not appear in the Not Ran Since Code Cleared list, the DTC P0300 diagnostic has run. If the MIL is NOT illuminated, and there is no Pending DTC Status in DTC Information, the DTC P0300 diagnostic has passed. The DTCs MUST be cleared in order to view the Current Status of the Not Ran Since Code Cleared list. DO NOT forget that the Not Ran Since Code Cleared list only indicates that the test has run, not whether the test passed or failed. The DTC Information screen must be checked for Current or Pending status, in order to determine the outcome of the diagnostic test involved.
Step | Action | Value(s) | Yes | No | ||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Schematic Reference: Engine Controls Schematics | ||||||||||||||||||||||
Did you perform the Diagnostic System Check-Engine Controls? | -- | Go to Step 2 | ||||||||||||||||||||
2 | Start the engine. Is a misfire present at idle? | -- | Go to Step 4 | Go to Step 3 | ||||||||||||||||||
Is a DTC P0300 set? | -- | Go to Step 4 | Go to Diagnostic Aids | |||||||||||||||||||
4 |
Was a repair necessary? | -- | Go to Step 18 | Go to Step 5 | ||||||||||||||||||
5 |
Did the test of each ignition coil indicate a crisp blue spark while cranking the engine? | -- | Go to Step 9 | Go to Step 6 | ||||||||||||||||||
Was a repair necessary? | -- | Go to Step 18 | Go to Step 7 | |||||||||||||||||||
7 |
Did the test indicate a crisp blue spark while cranking the engine? | -- | Go to Step 8 | |||||||||||||||||||
8 | Replace the faulty ignition coil. Refer to Ignition Coil Replacement . Is the action complete? | -- | Go to Step 18 | -- | ||||||||||||||||||
9 |
Did any spark plugs need replacement? | -- | Go to Step 12 | Go to Step 10 | ||||||||||||||||||
Did the fuel injector test lamp flash for each fuel injector while cranking the engine? | -- | Go to Step 11 | ||||||||||||||||||||
Check the fuel injectors for proper operation. Refer to Fuel Injector Balance Test with Special Tool and to Fuel Injector Solenoid Coil Test - Engine Coolant Temperature Between 10-35 Degrees C (50-95 Degrees F) . Was a faulty fuel injector found and replaced? | -- | Go to Step 18 | Go to Step 14 | |||||||||||||||||||
12 | Was the spark plug replaced because of a rich fueling condition or wet fuel fouling? | -- | Go to Fuel System Diagnosis | Go to Step 13 | ||||||||||||||||||
13 | Was the spark plug replaced because of oil or coolant fouling? | -- | Go to Step 15 | Go to Step 18 | ||||||||||||||||||
14 |
Was a repair necessary? | -- | Go to Step 18 | Go to Step 15 | ||||||||||||||||||
Was a basic engine mechanical concern found and repaired? | -- | Go to Step 18 | Go to Step 16 | |||||||||||||||||||
Was a repair necessary? | -- | Go to Step 18 | Go to Step 17 | |||||||||||||||||||
Was a repair necessary? | -- | Go to Step 18 | Go to Diagnostic Aids | |||||||||||||||||||
Does the DTC run and pass? | -- | Go to Step 19 | Go to Step 2 | |||||||||||||||||||
19 | With a scan tool, observe the stored information, Capture Info. Does the scan tool display any DTCs that you have not diagnosed? | -- | System OK | |||||||||||||||||||
